1. Field of the Invention
The present invention relates to a long-wavelength semiconductor light emitting device and its manufacturing method, and more particularly to those using GaInNAs-system semiconductors (simply called GaInNAs semiconductors hereinbelow) as materials of the active layer.
2. Description of the Related Art
Long-wavelength semiconductor light emitting devices using GaInNAs semiconductors as materials of the active layers can cover the emission wavelength region from 1.3 to 1.55 μm depending upon the mixing ratio of In and N in GaInNAs, and can be realized by using inexpensive GaAs substrates. Furthermore, these GaInNAs long-wavelength semiconductor light emitting devices permit large diffraction index differences An among layers of materials in lattice matching with substrates. Therefore, these materials make it possible to fabricate excellent distributed Bragg reflectors (DBR), and there has been a movement toward their applications to vertical cavity surface emitting lasers (VCSEL), which are hopeful as a form of optical communication lasers. Therefore, these GaInNAs long-wavelength semiconductor light emitting devices have been remarked for years as the next-generation optical communication semiconductor lasers substituting expensive GaInAsP long-wavelength semiconductor light emitting devices using InP substrates.
When a GaInNAs well layer is formed on an AlGaAs layer by metal organic chemical vapor deposition (MOCVD), the GaInNAs well layer catches Al therein by approximately 0.1% even though tri-methyl aluminum (TMA) is not supplied intentionally during its growth, and this aluminum adversely affects the static characteristics of the GaInNAs semiconductor laser. However, it has been reported that a GaInNAs well layer grown on a GaAs layer will not take Al therein (Photonics West 2003 Session No. 4995-08, herein below referred to as Non-patent Literature 1).
Additionally, there are some other proposals about techniques for manufacturing GaInNAs semiconductor lasers by the use of GaNAs layers as barrier layers of active layers (Japanese Patent Laid-open Publication No. JP-H10-145003-A, referred to as Patent Literature 1; Photonics West 2003 Session No. 4994-18, referred to as Non-patent Literature 2; and Photonics West 2003 Session No. 4994-33, referred to as Non-patent Literature 3).
In the above-introduced GaInNAs long-wavelength semiconductor light emitting devices, an AlGaAs layer in lattice matching with GaAs as its substrate is used as a clad layer. However, during the growth of the AlGaAs layer, tri-methyl aluminum or tri-ethyl aluminum (TEA) used as the source material of Al reacts in vapor phase with di-methyl hydrazine (DMHy) used as the source material of N, and produces reaction products (adducts). The Inventors confirmed by observation using a transmission electron microscope that the products of the vapor phase reaction produced during growth of the AlGaAs layer as a barrier layer of the active layer, for example, degrades the sharpness of the interface with the active layer.
Furthermore, the Inventors prepared a trial GaInNAs long-wavelength semiconductor laser and analyzed it by secondary ion mass spectroscopy (SIMS). As a result, they found a large amount of Al in portions of well layers and barrier layers even when any Al source material is not supplied intentionally into the reaction vessel during the growth of peripheral layers of the active layer (well layers, barrier layers and guide layers). Although the Inventors are not sure how the layers catch Al, they can presume that exposure of the substrate to an atmosphere containing a mixture of Al or its source material and an N source material invites the intrusion of Al and will cause a quality degradation of the active layer. This is a serious problem. A report of a research institute (Agilent Technologies) also remarks this issue of Al as inviting serious adverse influences to the static characteristics of GaInNAs semiconductor lasers (Non-patent Literature 1).
To overcome this problem, AGILENT proposes to first grow an n-type clad layer; then remove the substrate out of the reaction vessel of the growth apparatus; next clean the interior of the reaction vessel; and thereafter resume the growth of the active layer (Non-patent Literature 1). Taking account of defects departing from the interface of the layer grown after interruption of the growth, which will adversely affect the reliability, as well as an increase of the manufacturing cost by the need of the double-step growth, a new technique is invoked, which can prevent intake of the Al impurity in one step of crystal growth. In addition, for realization of practical GaInNAs long-wavelength semiconductor lasers, it remains unclear whether or not the Al impurity has to be removed completely from the active layer.